Wearable devices may provide interesting information, but the heart-rate data is unreliable and raises doubts about usefulness, a new study shows.

The new study, by cardiac experts at the Cleveland Clinic, examined four popular devices and found that their heart rate monitors are wrong 10%-20% of the time.

That may not matter much for the weekend warriors who just want to check on whether their fitness is improving. But a snapshot heart rate, even if accurate, won’t tell them much.

“On a day-to-day basis for an apparently healthy person, knowing your heart-rate does not provide any benefit,” said Marc Gillinov, a cardiac surgeon and one of the authors of the new paper, published in JAMA Cardiology.

“They’re selling millions and millions of these devices and so far we haven’t demonstrated a general benefit to them.”More than one in 5 Americans owns a wearable device like a Fitbit or Apple Watch, according to Forrester Research, Inc., though half stop using the device within six months of purchase.

Other recent studies also contradict the idea that the devices benefit health. One, published last month in JAMA, found that healthy young people who wore devices were less likely to lose weight than those who didn’t; another in The Lancet Diabetes and Endocrinology found that wearing a tracker did not encourage people to exercise more.

Positive readings seem to make people complacent and negative ones are so discouraging that people give up on their health goals, said John Jakicic, director of the Physical Activity and Weight Management Research Center at the University of Pittsburgh, and an author on the JAMA weight loss study. Instead of just relaying information, future devices should use proven science to encourage people to improve their health habits, he said.

Gillinov said he got interested in studying these devices two years ago, when his daughters both asked for Fitbits to track their heart rate. He took to the Internet to investigate which device would be most accurate and found very little data on any of them, he said.

He’s also had patients come to him talking of readings as high as 230 beats per minute – which would require immediate hospitalization, if true – and as low as 11, which would render someone unconscious if not dead.

“If you get a bizarre or almost unbelievable reading, don’t worry about it,” Gillinov said. “Check it once or twice more.”

Gillinov and his colleagues tested EKG and chest straps against four devices purchased in 2015: the Fitbit Charge, which now retails for $109.99-$129.99; the Apple Watch, which starts at $269; the Mio Fuse, which sells for $129; and the Basis Peak, which cost $199 before it was taken off the market in June because of a risk of burns.

Each of 50 volunteers wore two of the wrist-based devices, and walked, jogged and ran quickly on a treadmill for 3 minutes each.

The chest strap was nearly as accurate as an EKG and far more reliable than the wrist-based devices, the study found. The Mio Fuse and the Apple Watch returned the most accurate results of the four, correctly tracking heart rate 91% of the time on average. The Fitbit Charge and Basis Peak were accurate 84% and 83% of the time, respectively.

Chest straps are much more reliable for those who really need a heart-rate monitor, Gillinov said: elite athletes trying to push themselves to their limits, and cardiac patients who need to keep their exercise within strict boundaries.

The team is now studying more devices and with volunteers using different workout machines. His preliminary data, Gillinov said, suggest that frequent arm movements make the devices even less accurate.

One of the problems is that people don’t always wear the wristbands correctly. The devices work by sensing blood flow beneath the skin. If wristbands are worn too loose, they can’t accurately see the blood flow; if they are too tight, they constrict it.

Representatives of Apple Watch declined to comment on the study, referring readers to their webpage, which includes only general information about the devices.

In a statement, Fitbit said that it has tested its devices against chest straps and other devices and found that they perform "to industry standard expectations for optical heart rate on the wrist," with an average error rate of less than 6% or about 6 beats per minute. Monitoring is more reliable when taken over time, rather than as a snapshot, the statement suggested.

Mio Global, which makes the Mio Fuse, is getting ready to launch a new version of its devices that measures something called Personal Activity Intelligence, or PAI, developed by Norwegian cardiac researcher, Ulrik Wisløff. PAI, based on heart rate, is intended to be measured over time, rather than as a snapshot, and should give wearers a better means for controlling their health and fitness, said Liz Dickinson, the company’s founder and CEO.

“This is the decade for heart rate,” she said. “Absolutely, it’s coming.”

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My name is Meris. Through these articles, I would like to raise awareness of health and environment issues in order to better protect ourselves, our children and future generations. See also my Website in French Mieux Prévenir. For information in other languages, see my Facebook page.

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